Free radical reactions in purine analogs of DNA constituents will be studied by electron spin resonance (ESR) and electron-nuclear double resonance (ENDOR) techniques using single crystal samples. The analogs to be investigated have been shown to exhibit either oncogenic or tumor-inhibiting activities. Experiments with oncogenic purine-3N-oxides will be directed toward studying the precise electronic structures and the kinetics of primary and secondary radical species believed to be important in the oncogenic activity of these molecules. In addition the nononcogenic purine 1-hydroxyxanthine will be studied to examine possible free radical reactions involved in the photoisomerization of this molecule to the oncogen 3-hydroxyxanthine. Also because we have recently found evidence of a thermal-gradient-induced electric discharge in the oncogen 3-hydroxyanthine, we will investigate the possibility of radical reaction generation from this effect. Experiments with tumor-inhibiting mercaptopurines will be directed toward studying the effects of sulfur substituent alteration of radical reactions in purines which we discovered recently. Also experiments with the nononcogenic purine 6 mercaptopurine-3N-oxide will be performed to examine the sulfur inhibition of radical reactions expected to be important in the oncogenic activity of the purine 3N-oxides. Comparisons will be made between free radical reactions in oncogenic purines and in tumor-inhibitor purines to investigate correlations between these reactions and the tumor-activity of these molecules.